Generally, a PTFE porous material is manufactured by an expansion method and has a micro fibrous structure consisting of very thin fibers (fibrils) and nodes connected together with the fibers. The PTFE porous material is provided with a structure and characteristics as a porous material by such a micro fibrous structure, and the pore size, the porosity, etc. can be set to the desired values by controlling the expansion conditions, etc. The PTFE porous material obtained by the expansion method is called an expanded PTFE porous material (hereinafter, occasionally referred to as “ePTFE porous material”).
The PTFE porous material exhibits surface characteristics such as low coefficient of friction, water repellence, nonstickiness, etc. in addition to the characteristics such as heat resistance and chemical resistance which are inherent in polytetrafluoroethylene itself. Also, because of its porous structure, the PTFE porous material is afforded with characteristics such as flexibility, fluid permeability, property of uptaking fine particles, filtration property, low dielectric constant, low dielectric dissipation factor, etc. Since the PTFE porous material has such peculiar porous structure and characteristics, its use is expanding to the medical field in addition to the general field of industries.
For example, a tubular PTFE porous material is widely used as an artificial blood vessel for maintaining blood circulation, since it exhibits not only high flexibility and excellent antithrombogenicity that is inherent in polytetrafluoroethylene itself but also superior biocompatibility because of the porous structure based on the micro fibrous structure. That is, the artificial blood vessel made of a tubular PTFE porous material is widely used for transplantation or bypass at a lesion part of a blood vessel in a living body.
In recent years, for the purpose of decreasing the operative invasion into the human body, the method is developed in which a stent that can be delivered by a catheter and that has an elastic structure for shrinking/expanding in the radial direction is put in a blood vessel in such a manner as to expand a constricted cavity of the blood vessel. Such a stent is generally made of an elastic wire; the stent comprising an elastic wire made of metal is called a “metallic stent”. A stent that can be formed by covering such a metallic stent with a PTFE porous material has been developed. Such a covered stent can be used as an artificial blood vessel, i.e., a stent-graft or a blood vessel prosthetic device, which is made by providing a framework structural member (metallic stent) made of metal and capable of radial expansion/contraction with a covering material.
If such a covered stent is used, it is possible to cure an aortic aneurysm, for example, and its clinical application has already begun. However, in the covered stent, the technique for bonding a metallic framework structural member with a covering material made of resin is not established yet: there are cases where the metallic framework structural member of a metallic stent makes a hole in a covering material made of polyester or polytetrafluoroethylene by piercing through it, or causes the covering material to wear away by rubbing, which results in decreases of its strength or its breakage.
For combining a metallic stent and a covering material made of resin, methods are contrived in which the covering material is fixed to the metallic stent by stitching the covering material with a thread or by wrapping the inner and outer surfaces of the metallic stent with the covering material. Since a film of PTFE porous material tends to tear easily when it is stitched with a thread, there are proposed methods in which the inner and outer surfaces of a metallic stent are simply covered with a covering material, or in which the covering material and the metallic stent are bonded together with a plastic resin or the like disposed therebetween as an adhesive. See Japanese Patent Application Publication No. H7-24072, Japanese translation of PCT international Application Publication No. 2000-508216, Japanese translation of PCT international Application No. 2002-510985, and Japanese translation of PCT international application No. 9-501584.
However, since the technique for uniting a covering material and framework structural member together by bonding the covering material along the surface of each element (e.g., metal wire part) of the framework structural member has not been developed yet, there has been a shortcoming that a sufficient strength and durability cannot be achieved because of a crack that occurs between the covering material and an element of the framework structural member.
More specifically, in the case where a covered stent 144 has a construction in which a framework structural member is covered with PTFE porous membranes 141 and 143 as shown in a sectional view of FIG. 14(A), it is impossible to bond the PTFE porous membranes in close contact along the surface of each element of the framework structural member 142, which results in generation of cracks (cavities), although it is possible to bond the PTFE porous membranes together by making them to contact with each other at intervallic gaps between the respective elements of the framework structural member 142 if the pitch of an interval between the respective elements of the framework structural member 142 is sufficiently large. As shown in a partial sectional view of FIG. 14(B), even if an adhesive 148 is used, the PTFE porous membranes 141 and 143 are partially in contact with each other, and also partially contact each element of the framework structural member 142, whereby only partial bonding is achieved (for example, contact adhesion parts 147 and 145), allowing occurrence of gaps 146.
Moreover, it is extremely difficult to make the PTFE porous membranes to adhere along the surface of each element of the framework structural member by fusion or bonding by means of heating since the tension of a planar direction is increased because the PTFE porous membranes shrink by heating. Particularly, when a net-like structure, which is made by knitting metal wires at a narrow pitch of 3 mm or less, is used as a framework structure, it was almost impossible to make the PTFE porous membranes to adhere so as to be fixed to the surface of each complicated and thin element of the framework structure.
More specifically, as shown in a sectional view of FIG. 15(A), in a covered stent 154 having a configuration in which a framework structural member is covered with PTFE porous membranes 151 and 153, it is also difficult to bond the PTFE porous membranes 151 and 153 together partially in contact with each other at intervallic gaps when the pitch of an interval between the respective elements of the framework structural member 152 is small, and it is only possible to partially bond the PTFE porous membranes with the respective elements of the framework structural member. As shown in a partial sectional view of FIG. 15(B), even if an adhesive 157 is used, the PTFE porous membranes 151 and 153 are partially in contact with each element of the framework structural member 152 so as to be bonded therewith (155), and it is impossible to prevent the occurrence of gaps 156.
A conceivable method for preventing the occurrence of such a crack (cavity) is to fill a resinous adhesive fully in a space 164 between PTFE porous membranes 161 and 163 and a framework structural member 162 so as to unite them as shown in a partial sectional view of FIG. 16. Also, in the case of FIG. 17, it might be considered to fill a gap 174 with an adhesive when the pitch of the intervallic gap between the respective elements of a framework structural member 172 is small. However, since the adhesive buried in the gap between PTFE porous membranes 171 and 173 restricts the transformation of the framework structural member 172, the flexibility and the elasticity of the covered stent would be degraded. The covered stent without flexibility and expandable/shrinkable property will be unusable as a stent-graft that is required to exhibit a property of expanding/shrinking in a radial direction.
Japanese Patent Application Publication No. H7-24072 proposes a method in which a covered stent is manufactured by providing covering layers made of a PTFE porous membrane on both the inner and outer superficies of a tubular structure composed of elastic wires such as metal wires, and partially bonding the so-provided inner and outer PTFE porous membrane covering-layers together by hot-melt adhesion. However, the examples of this method include only a point adhesion or a line adhesion made at some parts of the PTFE porous membrane covering layers: there are no bonding made between a metallic stent and the respective PTFE porous membrane.
In the above-mentioned method, it might be conceivable that the PTFE porous membranes can be completely bonded together by hot-melt adhesion using a heat press machine and a mold with which the inner and outer PTFE porous membranes can be heated while a pressure is applied to the PTFE porous membranes. In this method, however, it is preferable to minimize an oppressive force applied from a lopsided local direction since the porous structure of the PTFE porous membrane tends to break from the part which is crushed by the oppressive force thus applied. Therefore, it is preferable to apply homogeneous pressure in the normal direction of the surface of the covered stent which is formed providing covering layers made of PTFE porous membranes on the inner and outer sides of the tubular structure composed of elastic wires. However, it is substantially only in one direction that a mold can apply homogeneous pressure to most part of the covered stent surface.
As concretely shown in a sectional view of FIG. 18, a covered stent, which is formed by providing covering layers made of PTFE porous membranes 182 and 184 over the inner and outer surfaces of a tubular structure 183 composed of elastic wires, is placed on the circumferential surface of a mandrel 181, and split dies 185-192 are arranged on the outside of the outer covering layer. Under such conditions, the inner and outer PTFE porous membranes are bonded together as a result of hot-melt adhesion caused by applying pressure and heat onto these split dies. In this method, the direction of the pressure of each split die is applied substantially in one axial direction. Moreover, the elastic wires, which are the constituting elements of the tubular structure 183, protrude, and therefore it is necessary to provide rather loose trenches 193 at the corresponding parts of the mold in order to prevent the porous structure of the PTFE porous membranes from being crushed or torn by pressure centering on the protruded parts (FIG. 19). However, in the case of the covered stent, it is substantially impossible to prepare a mold suitable for such a framework structural member or to adjust the positioning of the mold and the covered stent because in many cases the framework structural member has a shape formed by minutely complicated knitting. Moreover, as shown in a sectional view of FIG. 19, it is impossible to prevent the occurrence of crack (cavity) 194 even if an adhesive 195 is used in this method.
Because of the above-mentioned problems, the products for which a mold can be used is limited to those of smooth planar shape (plain film-like shape) or nearly equivalent to such a shape. Therefore, with a mold it is extremely difficult or substantially impossible to manufacture a covered stent having a cylindrical shape, tapered shape, bifurcated configuration, bow configuration, or a combination of these forms. Particularly, with a mold, it is practically impossible to manufacture a covered stent of custom-made form adjusted to the figure of a patient or the shape and size of a lesion part: much less those of 3-D asymmetry shape.
In order to firmly unite a framework structural member and a PTFE porous membrane together, it is preferable that a fluoroplastics layer such as an unsintered PTFE layer be interposed therebetween as an adhesive and the adhesive be fused by heating while pressure is applied so that they may be bonded together. However, in this method, since a mold for hot press must be heated to a high temperature of about 250° C. to 380° C., the applied heat tends to warp a mold, or cause the surface to oxidize, thereby making the mold fragile. Therefore, it is difficult to maintain the precision and the durability of the mold in the manufacture of industrial scale. Particularly, with a mold, it is extremely difficult to form thin PTFE porous membranes of 0.1 mm or less into a multiple layer.
Moreover, in the manufacture of a covered stent, the mold-releasing agent cannot be used for preventing residues from remaining in a product, and the consequent shortcoming is that the product tends to adhere to the mold heated at high temperature, thereby causing the product to tear at the time of removal from the mold. Such a problem is particularly great in the case of a product having a framework structural member of finely complicated shape in which the unevenness easily occurs.